Operation of a biogas plant in continuous regime is optimal from the viewpoint of maximizing biogas and fertilizer quantity as well as stability of digestion process and presumes daily substrate feed and discharge of fermented slurry.
Substrate feed tank
Fresh manure is normally collected in the substrate feed tank before uploading it into the digester. Depending on the type of plant the size of the tank should be equal to daily or double of daily volume of substrate input.
The tank is used to mix substrate and add water until it is homogenous and has the needed moisture content, sometimes automatic mixing equipment is used.
Sitting of the substrate feed tank
Situating the tank on the sunny side may promote preliminary heating of the substrate for immediate start of the digestion process after loading the substrate into digester. In cases when he plant is directly connected to stables the tank should be placed in a way so that the excrements flow directly to the tank aided by gravity.
Inlet and outlet openings
Inlet and outlet openings should lead directly to digester and are normally situated at the opposite sides of digester for homogenous distribution of fresh substrate along the whole digester and efficiency of discharge of fermented slurry. Inlet and outlet openings are made before mounting the digester and before staring of thermo isolation works.
For plants with underground digesters and manual loading of substrate inlet and outlet pipes should enter digester at an acute corner.
Both inlet and outlet openings should be situated with an inclination to vertical level in such a way that both inlet and outlet pipes will be drowned in substrate for keeping digester impermeable. In this way a hydraulic catch is created that prevents air from getting inside the digester.
Manual feed and discharge of substrate
The simplest method of feeding and discharging the digested slurry is overflowing when freshly loaded substrate increases the level of substrate in digester and pushes the same amount of digested slurry at the other end of digester to the biofertilizer collecting tank.
Loaded biomass may contain some particles of large size, for example bedding material (straw, sawdust), plant stems and other materials. For prevention of pipe clogging the diameter of pipes should be no less than 20 – 30 sm. Inlet pipe is connected with substrate feeding tank. Half-turn of spiral catches are installed n the inlet and outlet pipes.
Feeding and discharging of substrate with the help of pumps
Pumps become necessary parts of a biogas unit, when the amounts of substrate require fast movement and when gravity cannot be used for reasons of topography or substrate characteristics. Pumps are needed to compensate the difference in height between the levels of substrate feed tank and digester.
If it is not possible to avoid the use of pumps there are two methods of their installation:
• Dry installation: pump is installed together with the pipe. Substrate flows freely until it reaches the pump and is then accelerated.
• Wet installation: pump together with the motor is installed inside the substrate. Motor is placed in a liquid-tight container. Other option is when pump is working from the motor outside of substrate with help of shaft.
Pneumatic feed and discharge of substrate
Pneumatic method of loading and agitation of substrate is optimal. This method is used on all biogas plants constructed by PF “Fluid”. Pneumatic feeding system uses substrate mixing tank that is normally made of steel vessels of volume of 0,5 до 1 m3, that can bear the pressure of up to 5 bar, and pipes with diameter no less than 100 mm with a catch. Substrate is loaded to the mixing tank and from there into digester with the help of compressor.
For small and medium sized biogas plants with digester volumes up to 4 m3 the use of piston compressors IF-56 is recommended and for big plants with digester volumes above 50 m3 FU-12 compressors can be successfully used. Compressors also used for pumping biogas out of the digester.